The present invention relates to apparatus for heat sealing thermoplastic sheeting, e.g., so-called L-sealers, and, more particularly, to a novel arrangement for relieving tension in the sheeting during the sealing operation to achieve enhanced sealing strength.
In recent years, it has been increasingly popular and commonplace to package many commercial articles in close-fitting transparent plastic film. This packaging is accomplished conventionally by wrapping the article to be packaged loosely in a heat shrinkable plastic film, sealing together the abutting edges of the film to enclose the article, and subjecting the package to sufficient heat to activate shrinkage of the film tightly about the article.
Machines commonly referred to as L-sealers are available for commercially accomplishing the preliminary wrapping procedure as above-described. Heat shrinkable plastic film in continuous sheets pre-folded along a central longitudinal edge and packaged in convenient rolled form is utilized for this purpose. L-sealing machines characteristically provide a sealing table surface, usually an endless conveyor, for supporting the plastic film with the article inserted between the folds of the film and a pair of L-shaped jaws at the table adapted for receiving and grippingly engaging the film therebetween. One of the jaws is provided with a heated wire sealing arrangement to simultaneously sever and seal the film along a continuous L-shaped seal line extending parallel to the folded edge of the film and transversely between the parallel sealed extent and the folded edge. In continuous operation, the transverse seal formed in making each package provides the initial transverse seal for the next package so that each described L-sealing operation provides complete package formation. Representative examples of L-sealing equipment of the described type are found in U.S. Pat. Nos. 3,347,729; 3,490,981; Re. 30,010; 4,035,983; and 4,650,535. A more sophisticated L-sealing apparatus adapted for automatic high speed operation is disclosed in U.S. Pat. No. 4,219,988.
As will be appreciated, it is extremely important in the described L-sealing operation that a strong seal be formed uniformly and continuously along the L-seal line in order to insure proper shrinkage of the film about the package. The satisfactory accomplishment of this purpose has been an ongoing problem in the L-sealing industry for many years. Virtually all L-sealers utilized as one jaw a sealing bed fixed to the machine frame at the sealing table surface and as the other jaw a movable sealing arm pivoted to the frame and carrying the heated L-seal wires. Formation of the L-seal is a function of two basic criteria: (1) the proper alignment of the sealing bed and of the sealing arm with one another, and (2) the exertion of uniform sealing pressure along the entire L-seal line when the sealing bed and arm are closed together for sealing operation.
The aforementioned U.S. Pat. No. 4,650,535 discloses an improved L-sealing apparatus wherein an L-shaped sealing bed assembly is mounted at its ends and corner on three piston-and-cylinder assemblies operable upon closing of the sealing arm into adjacency with the sealing bed assembly to translatorily move the sealing bed assembly into uniform and continuous surface contact and pressure with the heating wire arrangement of the sealing arm. In this manner, this improved L-sealing apparatus automatically achieves self-alignment and uniform sealing pressure along the sealing bed and sealing arm to optimally satisfy the noted criteria.
While the apparatus of U.S. Pat. No. 4,650,535 achieves seals which are characteristically of superior strength to other conventional L-sealing apparatus, other factors may still affect and limit strength of the seal produced. In the normal sequence of operation of L-sealing apparatus, a machine operator inserts the article to be packaged between the facing webs of the folded plastic film and the film and package are moved into sealing disposition on the conveyor or other sealing table. Normally, this operation tautly tensions the opposed facing webs of the folded thermoplastic film to an extent that, if left tensioned, the film webs will resist movement into sealing contact with one another when the sealing arm is moved into sealing relationship with the sealing bed and thereby may inhibit or even prevent proper sealing of the film webs together. No conventional L-sealing machinery is known which addresses this particular problem. Instead, operators of conventional L-sealing apparatus are trained to pull a sufficient additional amount of film from its storage roll each time an article and film are moved into sealing position on the conveyor or other sealing table of the apparatus so that the extent of the film to be captured between the sealing bed and sealing arm is essentially untensioned. This technique is, of course, inherently unreliable and highly subject to variations in operator performance.